A typical positioning application to which the present invention relates is the positioning of a data transducer or "head" over a selected track of a magnetic disk file in a magnetic disk data storage system. Other uses will be evident to those skilled in the art.
U.S. Pat. No. 4,068,269 to Commander et al. discloses a transducer positioning system for a magnetic disk data storage system incorporating plurality of disks and associated magnetic transducers or "heads" for reading and writing data on each disk. The transducers are ganged for simultaneous movement by a single actuator. A single "servo" disk surface and associated "servo" transducer are dedicated to transducer positioning control. The dedicated servo disk surface is prerecorded with a plurality of concentric magnetic servo tracks of substantially uniform width which are arranged in alternate radial sectors and staggered radially in an alternating fashion from one another. Each magnetic servo track is provided with at least one change in direction of magnetization. As the servo disk surface is rotated, the servo transducer generates a signal indicative of the magnetic transitions occurring in the servo tracks opposite it. The transducer generated signal is passed through appropriate circuitry which generates a first or "normal" position signal and a second, "quadrature" position signal. The position signals are oscillatory about a zero voltage and 90 degrees out of phase with one another. Each of the two position signals is associated with one of the two alternating sets of servo tracks. Each of the position signals is linear for approximately one quarter track width to either side of the boundary of adjoining tracks in the set of sectors with which the signal is associated. The two position signals are alternately linear as the servo head is moved radially across the servo disk surface. The normal position signal, which is selected to be linear about each on-data track position, is used for transducer position control during track following operations when data is being read onto or from the disks.
A positioning system must also control transducer movement between data tracks (and corresponding servo tracks) in an "access" or "seek" operation. The time taken to move the heads between selected tracks in such a mode is generally known as the "access" time and is one of the more important performance characteristics of the positioning system. To minimize the access time for a given mechanical configuation and actuator performance requires a positioning system which will control head movement velocity at an optimal level and bring the transducer accurately to rest on the desired track.
In the aforesaid U.S. Pat. No. 4,068,269, access motion by the transducer is accomplished by means of a continuous distance to go signal generated by counting down the number of tracks between the original position of the servo transducer and desired position of the servo transducer using track crossing pulses generated by logic identifying the alternating linear portions of the normal and quadrature position signals. The derived distance to go signal is passed to a reference velocity signal generator which outputs a time-optimal reference velocity signal which is compared with the actual head velocity signal derived by differentiating and piecewise combining the successively linear portions of the normal and quadrature position signals.
In U.S. Pat. No. 4,115,823, also to commander et al., there is described yet another positioning system for use with a disk data storage apparatus similar to that just described wherein the normal and quadrature position signals generated by a dedicated servo transducer and disk surface are combined with servo position signals generated by a data transducer from servo signals mixed with data signals on the data disk surface. Again, the linear portions of the two position signals are alternatively differentiated and combined to generate a velocity signal used in head control.
There are several limitations associated with the positioning systems described in the U.S. Pat. Nos. 4,068,269 and 4,115,823. First, only one-half of the available servo position information is utilized as only the linear portions of the normal and quadrature position signals are used in positioning the transducer. Next, servo track widths are identical to data track widths. As data track widths are made narrower by various techniques to increase data density, the servo tracks must be similarly narrowed. As the servo tracks are contiguous and extend entirely across the servo disk surface, this becomes more expensive to accomplish. Moreover, as data and servo tracks are recorded with narrower widths, the described positioning systems become more susceptible to mechanical perturbations such as eccentricity which may drive the servo transducer into the non-linear region of the normal position signal or trip the transducer onto an adjoining servo track.
Other disadvantages arise in the described systems. Accurate positioning becomes difficult during transducer movement because noise in the system becomes predominant when the positional signal is differentiated at low velocity, as when the transducers are approaching their final position. In differentiation type systems such as have been described, variations in the level of the position signal can similarly cause difficulties. Such variations may be caused by fluctuations in transducer fly height with respect to the disk. As a result, the smoothness of the disk's surface must be held to very tight tolerances. Each of the described Commander et al. systems is also sensitive to position signal wave form linearity. Any deterioration of the servo head or associated electronics can effect the linearity of the positional signal wave forms and cause serious control problems. This requires the imposition of stringent manufacturing tolerances with respect to the components associated with the servo control system.
Bandwidth requirements imposed on positioning control systems during access-type operations may be significantly reduced by the use of feedforward control. U.S. Pat. No. 4,200,827 to Oswald describes a feedforward/feedback transducer positioning system used in a magnetic disk data storage device. In feedforward control, a primary current is applied to the actuator moving the heads. The primary current is one which would accomplish an optimal or near-optimal movement of the heads in an ideal or nominal model of the electromechanical servo mechanism being used. Variations between the actual performance of the system and the modeled or ideal performance upon which the primary current is based is compensated for by introducing small perturbations into the primary current as feedback control.
U.S. Pat. No. 4,200,827 describes a "bang-bang" access servo control system in which the heads are moved by the control system at near the maximum acceleration and deceleration which the electrical and mechanical components of the system can tolerate. For long access movements, the heads "coast" at maximum velocity between acceleration and deceleration modes. The control system generates a drive current (or feedforward current) which can be controllably switched in sign for movement of the heads in either direction along a radius of the disks. Before being fed to the actuator motor moving the heads, the drive current is combined with a feedback control current proportional to error occuring in the access operation. Two embodiments are described, one utilizing velocity error and the other utilizing position error to generate the feedback control current. In the former embodiment a transducer head velocity signal is generated by differentiating a single cyclically varying servo position signal (i.e. normal position signal) generated by a dedicated servo transducer and associated servo disk surface. During the non-linear portions of the servo position signal, actuator current, which is proportional to acceleration, is integrated and used as a measurement of velocity. In the latter embodiment, one or two periodic servo head position signals (i.e. normal or normal and quadrature signals) are generated by the dedicated servo head and associated servo disk surface. A reference position signal is generated by integrating a reference velocity signal. The reference position signal is then phase compared with the servo head position signal to generate a position error signal and a proportional position error current which is combined with a drive current, as in the velocity controlled system, to provide a varying current to the actuator. In a preferred embodiment of the position error control system, both normal and quadrature servo head position signals are generated and the linear portions of each phase compared in a sequential, alternating fashion (as was done in the Commander '269 and '823 patents) with corresponding normal and quadrature reference position signals. The latter are generated by integrating and then phase shifting a single velocity reference signal.
As the invention of the U.S. Pat. No. 4,200,827 patent determines servo position by combination of normal and quadrature signals in the manner of the two aforementioned Commander et al. patents, it suffers the same drawbacks. Additionally, where velocity error is used as the feedback control mode, actuator motor performance must be tightly controlled to predicted nominal conditions or errors are introduced to the measured head velocity signal generated by integration of the motor current. This error is cumulative during each access operation and makes landing on track at the end of the operation problematical at best. As a result, very tight manufacturing and reliability tolerances are imposed on the actuator motors which must be used with this system.
U.S. Pat. No. 4,297,734 to Laishley et al. describes yet another servo positioning system for data disk systems utilizing feedforward plus feedback control with sampled, rather than continuously generated normal and quadrature servo position signals. This control system is subject to the same problems which beset the previously identified systems, particularly the requirement that actuator motor performance be tightly controlled during manufacture and monitored over the life of the system. As servo position is only periodically sampled and not continuously monitored, small variations of the actual actuator motor performance from modeled motor performance can introduce significant errors degrading feedback control.